The irradiation of certain sample materials with an intense monochromatic light causes pertubations of the molecular energy levels of the sample. As a result, secondary light waves with wavelengths which are different than that of the monochromatic light are produced and radiated from the sample. This phenomenon is known as Raman scattering.
Raman spectroscopy is a well established technique that enjoys wide application in industry and many fields of research. However, Raman scattering is a very weak effect. Consequently, conventional Raman spectroscopy requires a highly focused and intense laser beam and a very efficient detection system. Typically, Raman light is collected from a point source, within a small solid angle.
In 1972, Stone and Walrafen demonstrated that the use of a liquid core waveguide can enhance the Raman signal detected from a sample by a factor of 10.sup.2 to 10.sup.3. This improvement resulted from using a sample solution as the core and the cell wall as the waveguide cladding. Accordingly, both the excitation light and the Raman light are guided along the waveguide/cell and thus through the liquid sample. Increasing the length of the tubing increases the number of interactions between the excitation light and the sample solution, thereby magnifying the Raman signal. Stone and Walrafen used quartz tubing as their cell and were unable to measure Raman spectra in fluids having refractive indices equal to or lower than the refractive index of quartz (1.46). Restated, the described technique relied upon reflection from the interface between the sample/core and the cell wall and the requisite reflection, in turn, requires that there be an interface between the core and a material having a lower index of refraction,
In 1987, Schwab and McCreery disclosed a long capillary Raman cell consisting of uncoated glass tubing exposed to air. Total reflection theoretically occurred at the glass/air interface on the exterior surface of the tubing. With a refractive index of 1.0, the air total reflection surface virtually removed the constraint on the refractive index of the liquid sample. However, this design is subject to several limitations. The excitation light intensity is almost evenly distributed within the total diameter of the cell. Consequently, the cell wall must be very thin to retain effective excitation intensity within the sample. Therefore, cells in accordance with this design are exceedingly fragile and are likely to break when subjected to a slight amount of flexing. Furthermore, the outside of the cell must be kept extremely clean in order to maintain efficient waveguide action.
Due to the above-discussed limitations of the previously proposed apparatus, the liquid core waveguide method for intensifying Raman signals has not been exploited. The failure to exploit this technology can be primarily attributed to the unavailability of an appropriate material with which to construct a commercially viable aqueous waveguide cell.